This invention relates to a mixer particularly adapted for heating and cooling for use in chemical process applications. The mixer has a symmetric mixing element which comprises a plurality of mixing rods which are parallel to the mixing container axis and the rotation axis of the mixing element. The mixing element follows a planet-like movement about the container axis; the direction of rotation of the element is opposite to that of the rotation of the element about its own axis. Such a mixer is shown in German Pat. No. 344,764.
The prior art mixer shows a revolving mixing container having two cage-like mixing elements which are rotatably attached to two rigidly mounted arms. A gear rigidly mounted on the container, drives the mixing element via an intermediate gear located on the arm. The direction of movement of the element about its own axis is the same as the direction of movement of the container about the container axis.
Those skilled in the art were previously of the opinion that, in order to achieve particularly good mixing, the speed of revolution of the mixing element should be relatively high in comparison with the speed of revolution of the planet-like movement with which the element moves about the container axis. In the case where such a mixer is used as a heating mixer, where the warming effect is produced by means of friction in the mixing material itself and between the mixing material and the mixing element, the mixer will be limited to its effect by means of the warming of the outer extremity of the mixing element.
The center of the mixing element will remain relatively cool causing a non-uniform heat distribution in the mixing element and in the mix material. This is particularly unacceptable in the case where the warming is to bring about a chemical or physical transformation since this will cause the transformation to be non-uniform.
According to the instant invention, a mixer is disclosed in which a constant mixing pattern is achieved over the total cross-section and over the total longitudinal section of the mix container; that is, on all positions where mixing material contacts the mixing elements, a constant velocity is obtained, thereby causing a uniform warming of the mix material and the mixing element.
In order to accomplish this, the angular velocity must satisfy the condition .omega..sub.P = .omega..sub.M wherein .omega..sub.P is an angle of velocity of the mixing element about the mixing container axis and .omega..sub.M is the angle of velocity of the mixing element about its own axis. If one would watch the movement of the mixing element in the mixing container, it would appear that the mixing element did not revolve at all. Any particular point located on the mixing element describes a circular path about the container axis. This is the reason that the usual propeller or rake is not used as a mixing element, since this could not cover the total container cross-section. The mixing element advantageously comprises a plurality of rods which are preferably regularly distributed about the circumference of an element and are located about the rotational axis of the mixing element.
A mixing element turns about its own axis so often as it is turned about the container axis; no prior art device operates in this fashion. In the case of a revolving container, the container speed of revolution equals the speed of revolution of the mixing element. Up to now, it was not known that, by this means, all particles of mix material throughout the container will contact the rods of the mixing elements at the same speed. In the mixer shown in the afore-mentioned DT-PS No. 344,764, this is not the case, since this mixer has a mixing element which rotates at a much higher speed than the container.
If the mixing element is to be used to actively temper the mixing material, an appropriate apparatus can be provided, e.g. the rods of the mixing elements can contain electric windings or a heating or cooling medium can be circulated through the rods and be connected to an appropriate circulation system. Since the mixing element according to the invention rotates about its own axis only one time for every single planet-like circulation, the driving means for the elements can be of particularly simple construction.
A simple driving connection between the driving axis of the mixing element and the axis of the container will suffice. The axis of the mixing element and the axis of the container can each carry a gear, the gears being connected by a free-running connecting gear and having the same amount of teeth; the connecting gear will be carried by the same means which connects the container axis to the mixing element axis. It is also possible to locate a toothed pulley on each axis, each pulley having the same diameter and being connected by a toothed belt. This belt causes a movement in which the axis of the mixing element does not turn relative to the axis of the container during the planet-like movement of the mixing element about the container axis.
With respect to this embodiment, the difference between the inventive concept and known mixer is particularly clear. As has already been mentioned, there is produced a uniform mix pattern over the total container cross-section. This is an important advantage over the prior art. Another important advantage is that, with the same container dimensions and the same contact velocity the kinetic energy which is released for warming is three-times higher with the mixer of the invention as in the prior art mixers.
Another advantage of this invention is that the mixing element contacts the mixing material equally with all of its surfaces; therefore, there will be no shadow-like build-up of mix material deposits on the mixing element. A disadvantage generally found in the prior art.
Although the main advantage of the invention is seen in the case where one wishes to achieve a uniform warming of the mixing material, the invention can also be utilized in the case where a uniform mix pattern is desired, e.g. in case of mixing volatile liquids where one seeks to disturb the upper surface as little as possible. For the mixing of this type of material, one can utilize a mixer with a mixing element which is inclined with respect to the container axis. The speed of revolution is adjusted so that .omega..sub.M = .omega..sub.P . cos .alpha. where .alpha. is the angle between the axis of the element and the container. One would not obtain a uniform mixing pattern in the direction of the container axis; however, one would achieve this in the direction perpendicular to the container axis. This type of mixer also has an improved mixing pattern with respect to the similar known mixers.
If a more highly viscous mass, for instance a gel or paste, is prepared with such a mixer, this mass only inadequately flows downwards from the container walls, and some of it may become baked on.
According to a further development of the invention, therefore, at least one wall scraper is arranged inside the mixer which rotates around the mixing container axis in the same direction as the mixing element, but with at most half the angular velocity compared to the latter.
These measures ensure that the scraper does not encounter the mixing material at a higher velocity than that of the individual stirring rods of the mixing element. It is expedient to go to the upper limit of what is permissible, i.e. to have the wall scraper rotating about the container axis with half the angular velocity of the mixing element. The scraper, which also creates warmth in the mixing material by means of friction, then has the optimum effect.
Further novel features and other objects of this invention will become apparent from the following detailed description, discussion and the appended claims taken in conjunction with the accompanying drawings.